Centrifugal elutriation has been adapted for platelet separation from human blood. Beckman chamber was tested and found to be inadequate for this purpose. Several new chambers of different design were built and their fluid dynamics analyzed. A chamber of parabolic shape with a spread out top was found to be effective for platelet separation. Using this chamber, platelets have been isolated in high yields, up to 95% total recovery. The average peak concentration of the eluted platelet fractions reached 304 x 10 to the 6th power cells per ml in 8 ml. The isolated platelets appear to be free of red and white blood cells and morphologically intact. Serotonin secretion and aggregation properties of these platelets were found to be similar to those in platelet-rich plasma serving as control. These findings support our proposal to build a larger system. We expect that it will provide pure and functional platelet concentrates suitable for clinical transfusion requiring shorter processing time and involving lesser cost than presently used systems allow.